Automatic impurity profiling machine

ABSTRACT

A MACHINE AND METHOD FOR AUTOMATICALLY DETERMINING THE DISTRIBUTION OF IMPURITIES CONTAINED IN A SEMICONDUCTOR BODY. THE MACHINE IS PARTICULARLY USEFUL IN DETEMINING THE DEPTH OF VERY SHALLOW JUNCTIONS IN THE ORDER OF 1/2 MICRON AND LESS. THE MACHINE SUCESSIVELY ANODIZES THE SURFACE OF THE SEMICONDUCTOR BODY TO PRODUCE AN ANODIZED SEMICONDUTOR LAYER, REMOVES THE ANODIZED LAYER BY CHEMICAL ETCHING, MEASURES AN ELECTRICAL CHARACTERISTIC, SUCH AS A SPREADING RESISTANCE AND SHEET RESISTANCE, OF THE SEIMICONDUCTOR BODY AND RECORDS THE ELECTRICAL CHARACTERISTIC IN TERMS OF THE CHARACTERISTIC VERSUS THE AMOUNT OF MATERIAL REMOVED. THE MACHINE AUTOMATICALLY REPEATS THIS CYCLE UNTIL A RESISTIVITY PROFOLE FOR THE DESIRED DEPTH OF MATERIAL IS OBTAINED. A SHARP CHANGE WILL BE OBSERVED IN THE RECORDED PROFILE OF THE ELECTRICAL CHARACTERISTIC VERSUS THE AMOUNT OF MATERIAL REMOVED WHEN A PN JUCTION IS CROSSED.

1. 1;, 1971" M.c.buFr-'Y, E 5

E f AUTOMATIC IMPIIJRITY PROFILING MACHINE Filed Dec. 22,1196? s Sheets-Sheet 1 'ELECTROILYTE INTO ANQDIC OXIDATION L lNlT EL cTlioLYs'ls E H S 1 f I C RQL IE qxmlzso A E I REMOVAL 0 v sP (SPREADING RESISTANCE).

, v x (AMOUNT REMOVED) RINS E&DRY E PROBE powN- FIRST READING DATA RECORDING umr REVERSE CURRENT SECOND READING PROBE UP BEGIN INVENTORS NEXT CYCLE MlCHAEL c DUFFY ATTORNEY Jan. 12, 19-71 v M. c. DUFFY ETAL 3,554,891 I 1 AUTOMATIC IMPURITY PROFILI-NG'MACHINE FiledDec. 22, 1967' i. s Sheets-Sheet 2 FIG. 4

12,1971 Meow-1 v em I 3,554,891

v AUTOMATIC IMPURITY PROFILING MACHINE iled Dec. 22, 1967 I 5 Sheets-Sheet s ELECTROLYTE AEITCHANTY FIG.6

AUTOMATIC IMPURITY PROFILING MACHINE Filed Dec. 22, 1967 I 5 Sheets-Sheet 4 @ELECTRICAL ANODIC, I DATA.

CONTROL OXIDATION RECORDING umy- UNIT UNIT\ A l FIG 7 u 2s0|(- 2A ,FIGQ

- a: Fla-l $.l Iw' w-10K-- o I: o 1 2 3,4 s-s MATERIAL REMOVED (MICRO .mcuas) FIG. 10 1600- 400- C 0200" Q I CL MATERIAL REMOVED MICRO INCHES M. Q DUFFY ETAL AUTOMATIC IMPURITY PROFILING- MACHINE 5 Sheets-Sheet 5 Filed Ded. 22. .1967

. q .wtzz zomkmwz M636 Z33 (.100. 1 VOLTAGE (VOLTS) v a m 0 w 20 00 5600 MATERIALIREMQVED (ANGSTROM UNITS) United States Patent rm. Cl. B01k 3/00 US. :1. 204-195 6 Claims ABSTRACT OF THE DISCLOSURE A machine and method for automatically determining the distribution of impurities contained in a semiconductor body. The machine is particularly useful in determining the depth of very shallow junctions in the order 1 of /2 micron and less. The machine successively anodizes the surface of the semiconductor body to produce an anodized semiconductor layer, removes the anodized layer by chemical etching, measures an electrical characteristic, such as spreading resistance and sheet resistance, of the semiconductor body and records the electrical characteristic in terms of the characteristic versus the amount of material removed. The machine automatically repeats this cycle until a resistivity profile for the desired depth of material is obtained. A sharp change will be observed in the recorded profile of the electrical character'- istic versus the amount of material removed when a PN junction is crossed.

BACKGROUND OF THE INVENTION 1) Field of the invention The present invention is directed to arnethod and apparatus for determining the characteristics of semiconductor devices, more particularly a method and apparatus for the determination of the distributions of impurities within a semiconductor device. Themethod and the apparatus produces aresistivity profile which can be convertedto a concentration profile within the semi conductor and allows the accurate measurement of even shallow junction depths and the determination of the fine details of the impurity distributions in semiconduc- .tor junction structures. r

(2) Description of the prior art and stain technique is that for very shallow junctions, such as less than /2 micron, it is impossible to get accurate junctiondepth measurements. Further, this work must be done by hand, is very tedious and time consuming even to the most skilled technician.

SUMMARY OF THE INVENTION the impurity profile of a semiconductor body.

It is another object of this invention to provide a method for determining the depth of a PN junction in a semiconductor body by automatic procedures.

" It is thus an object of the present invention to provide a method for automatically determining Patented Jan. 12, 1971 It is a still further object of this invention to provide a completely automatic process for determining the impurity profile and the PN junction depth of a semiconductor device.

These objects are accomplished in accordance with the broad aspects of the present invention by providing an automatic procedure which successively anodizes the semiconductor body to produce an anodized semiconductor layer, remove the anodized layer, measure an electrical characteristic of the semiconductor body and records the electrical characteristic in terms of the characteristic versus the amount of material removed. This procedure is repeated automatically for the desired depth of the material to be profiled.

The impurity profile is then used to characterize the semiconductor body and to determine depths of the PN junctions within the semiconductor'body by observing any sharp changm in the impurity profile. The anodizing technique allows the controlled removal of a very small quantity of semiconductor material. Prior art methods, such as chemical etch and mechanical polishing, could not accurately control the removal of quantities of semiconductor much less than about 5000 angstrom units. The anodizing method can remove semiconductor layer thicknesses of as little as angstrom units conveniently.

The impurity profile measuring apparatus includes a container having input and output ports. Within the container the semiconductor is supported in such a fashion as to make a semiconductor substrate the anode of the apparatus. A suitable cathode contact is positioned within the container. A device for measuring an electrical characteristic is positioned in such a manner as to allow its function at the appropriate time within the methods cycle. The measuring device has an output to a suitable recording means which is programmed for recording the electrical characteristic in terms of the characteristic versus the amount of material removed. Control means cause the automatic cycle of the input and output of anodizing solutions into and out of the container, the application of an anodizing potential between the anode and cathode while the anodizing solution is in the container, the input and output of an etching solution following the output of the anodizing solution; the means for measuring makes a measurement of the electrical characteristic and the means for recording record the measurement.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more'particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a greatly enlarged cross-section of a semiconductor device showing two P-N junctions;

FIG. 2 illustrates a typical electrical characteristic profile which corresponds to the FIG. 1 semiconductor device structure;

FIG. 3 is a flow diagram of one form of the process of the present invention; 1

FIG. 4 is a side view of a preferred anodizing unit of the impurity profile measuring apparatus;

FIG. 5 is an enlarged cross-section of the anodizing container portion of the FIG. 4 impurity profile measuring apparatus;

FIG. 6 is a top view of the container portion of the FIG. 4 impurity profile measuring apparatus;

FIG. 7 is a block diagram of an impurity profile measuring apparatus;

FIG. 8 is a graphical representation showing silicon dioxide thickness versus voltage applied;

FIG. 9 is a graphical representation of sheet resistance versus material removed;

FIG. 10 is a graphical representation of photovoltage versus material removed; and

FIG. 11 is an impurity distribution profile comparing the results of the method of the present invention to the radio tracer neutron-activation method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more particularly to FIGS. 1 and 2 the process of the invention may be readily understood. FIG. 1 illustrates in an enlarged cross-section, a NPN semiconductor structure 10 having an N -P junction characteristic of an emitter base junction 12 and a P-N junction characteristic of a base collector junction 14. FIG. 2 shows the spreading resistance profile curve of the FIG. 1 structure which results from the method of the present invention. The FIG. 2 resistance profile has the electrical spreading resistance characteristic of the semiconductor structure recorded versus the amount of material removed. At each P-N junction, the spreading resistance increases very rapidly resulting in sharp peaks which are shown at 16 and 18 which represent the depths of X,-,,, the emitterbase junction, and the X the base-collector junction.

The flow diagram given as FIG. 3 illustrates the steps in the process used to obtain a resistivity profile such as shown in FIG. 2. The semiconductor structure to have its impurity profile detenmined is placed in the anodic oxidation unit. The semiconductor structure is attached to the positive terminal of a variable DC supply voltage. Another electrode is suitably placed so that it will be subsequently covered with electrolyte. The electrolyte is then flowed into the anodic oxidation unit. The application of the voltage in the suitable electrolyte causes the growth of an oxide on the semiconductor device surface. The amount of this growth of oxide is controlled by the time in the electrolyte and the particular electrolyte utilized and voltage applied. When the desired increment of oxide is obtained the voltage is removed and the electrolyte is exhausted from the oxidation unit. The oxidized layer is then removed by the use of a suitable etchant. The semiconductor is then rinsed and dried. A spreading resistance or sheet resistance probe is then applied to the surface of the semiconductor device and the resistance measured. A sheet resistance measurement by the four-point method is the most commonly used method for resistivity movement. The spreading resistance measurement by a threepoint probe is a simplification of the four-point method. (Reference: Fundamentals of Silicon Integrated Device Technology, Vol. I, Edited by R. M. Burger and R. P. Donovan, pages 421-422, Prentice-Hall series.) A reverse current resistance reading is then obtained using the resistance probe. The two resistance readings are electrically recorded in a data recording unit. The resistance probe is then raised and the cycle repeated. The cycle is repeated many times until the desired amount of semiconductor material is removed. The removal is in small increments such as, for example, 100-300 angstrom units per cycle. A spreading resistance profile such as shown in FIG. 2 is the result of this automatic recycling process.

The anodic oxidation unit is illustrated in FIGS. 4-6. The unit includes a container 20 having input ports 22 and output or exhaust port 24. A probe 26 for measuring the spreading or sheet resistance of a semiconductor device mounted within the container 20 is capable of being moved into and out of the container 20. A probe arm 28 supports the probe. Clamp 30 holds the probe arm 28. Further suitable mechanical linkage (not shown) connects the probe arm to the motor 32 which drives the probe into and out of the container 20. An up and down adjustment micrometer 34 is used to make a fine probe adjustment. The dashed lined structures and numbers having primes indicate the structural elements of the probe and supporting structures which are fully raised out of the container 20. The solid lined structures and the unprimed numbers correspond to the probe structure fully inserted into the container 20. An indexing motor 36 is provided with suitable mechanical linkage (not shown) which drives the probe into and out of the paper in very small increments. The micrometer 38 allows the fine adjustment of the probe from left to right on the surface of the drawing.

FIGS. 5 and 6 show the detailed construction of the anodizing container 20. 20 includes the anodizing cup 40 having a clamping insert 42. At the bottom of the anodizing cup is a copper electrode block 44 which is appropriately electrically connected as anode of the cell by electrical conductor 46. The clamping insert 42 has clamping structure 48 which holds the semiconductor device 50 onto the upper electrode 44 securely. A washer 52 which may be composed of, for example, a polyvinyl material can additionally be inserted between the clamping insert 42 and the electrode 44 to form an additional portion of the clamping structure. The clamping insert 42 has suitable openings which align with the ports 22 and 24 in the anodizing cup 40 to allow the suitable input and output flow of liquids and gases to and from the open area 54. A suitable electrode 56, which may be composed of a noble metal such as platinum, is positioned within the area 54 so that the electrode will be submerged in the electrolyte. This electrode is connected as the negative terminal of the anodization cell. The electrode 56 is suitably insulated from the container 40 by insulating material 58.

The probe 26 is illustrated as having four contact points 60 useable in taking the electrical measurements. The four point probe 26 is electrically connected by means of conductor 62 to the data recording unit. FIGS. 5 and 6 illustrate by arrow and identification the materials which flow into the container 40 and which are exhausted therefrom. It is therefore seen that the input ports include the electrolyte input, the deionized water rinse input, the cleaning solvent input, the etchant input and the drying gas input. In addition there is the container exhaust output. The connecting tubings which connects the source of the fiuids and vapors that flow into and out of the container 40 to the input and output ports 22 and 24 are not shown for simplicity sake.

FIG. 6 additionally shows an arm release mechanism which allows the swinging of the probe arm completely out of the way of the top opening to the container 20. This includes a joint 82 in the probe arm and a screw clamp 84 which holds the joint rigid until it is desired to swing the probe arm 28 and the attached probe 26 completely out of the way of the top container 20 opening. The dashed lined structures and numbers having primes indicate the structural elements of the probe and arms structures which are fully swung away from the container 20. The solid lined structures and the unprimed numbers correspond to the probe and arm structures in their normal position.

FIG. 7 shows the overall structural connection between the electrical control unit 90, the anodic oxidation unit 92 and the date recording unit 94. These are the major elements of the impurity profile measuring apparatus. The electrical control unit is the means for controlling the input and output of the anodizing solutions into the container, the application of an anodizing potential between the anode and cathode of the cell while the anodizing solution is in the container, the means for causing the measurement of the desired electrical characteristic of the semiconductor device under test, and the means for causing the recording of the measured electrical characteristic. Further, the control unit 90 causes the continuous cycling of the process until the desired impurity profile is obtained in the data recording unit. In the preferred The electrical control unit 90 is a electrical circuit composed oftimers, relays and electrically operated'walves which cause the sequential performance of the cycle described above. The details of the circuit are not included for simplicity sake. I j

The data recording unit 94 iscomposed of a 3440A 'HwlettPackard Digital Voltmeter connected to a H-P 62ADigital Recorder. When the'voltmeter is triggered,

theoutput of'the voltmeter is fed into the recorder which records the reading the form of sheet resistance, spreading resistance or photovoltage versus material removed. The profile as FIG. 1 1 is'not directly dete mined from the'data' The data is fed into a computer (not shown) whichin'turn converts the data to an impurity profile. Y J Semiconductor" devices composed .of silicon germanium can readily have their junction profilesobtained according to the automated methdd'o'f the present invention. There are a variety of suitableanodization electrolytes for both silicon and germanium. Examples for silicon are nitric acid (HNO ethylene glycol, tetrahydrofurfurgyl alcohol with sodium nitrite (NaNO An example for germanium is hot water. The preferred electrolyte for silicon is a mixture of N-methylacetamide, potassium nitrate and .water. An example-for the silicon'dioxide is hydrofluoric acid (HF). p v p The sheet resistance measurements'is 'madeby the four-point probe method and spreading resistance measurement is made by'the three-point probe metho d as' illustrated by the flow .diagram and the measuring apparatus of FIGS. 4 through 6. The sheet resistance..R ,,is related to the concentration of impurity by the following equation:

when q is the electron charge, u the eifective carrier mobility, Tx, the conductivity and Cx is the concentration of impurities. A computer program is utilized in the comvputation of the concentration values as a function of mat'erial removed. (Reference: Detailed Analysis of Thin Phosphorus Diffused Layers in P-type Silicon, E. Tannenbaum, Solid State Electronics 2, March 1961, pp. 123- 132.)

A photoelectric technique can .be used to replace the spreading resistance measurement. This procedure used the rectifying properties of semiconductor junctions. T wo probes, with a high impedance voltmeter connected across them are brought down onto the surfaces of the wafer which has been illuminated with a light source.

.The light induced current is read on the voltmeter. Depending upon the impurity type of semiconductor matefrial, a certain polarity of current is indicated on the voltmeter. Once the junction is passed by the removal process of the present invention, the observed polarity reverses. The junction is as the point of reversal of the polarity of the current. The graphical plot of the photovoltage as a function of the amount of material being removed, provides the position of the junction in the semiconductor structure.

Where a silicon semiconductor structure is to be profiled, the preferred electrolyte is a mixture of millimeters of N-methylacetamide, 1 gram of potassium nitrate, and 25 ml. of de-ionized water. With the silicon device connected as the positive terminal of the anodization cell and a platinum electrode immersed in the electrolyte as the negative electrode, the application of a suitable voltage across the cell produces a growth of silicon dioxide on the surface of the silicon material. This anodization reaction proceeds to an oxide thickness gOV- erned by the voltage applied. FIG. 7 shows the difference between the silicon dioxide thickness for P-type doped silicon and a N-type doped silicon device in the range of 100 to 1300 angstrom units of silicon dioxide as a function of applied voltage.

The following examples of the present invention are included in order to aid in the understanding of the invention and variations may be made by one skilled in the art without departing from the spirit and scope of the invention.

EXAMPLE 1 Three N-type silicon semiconductor wafers were placed in a suitable difiusion capsule. A boron source of 2.5 X 10 atoms/cc. was placed in the capsule. Diifusion of the boron source into the silicon N-type wafer was accomplished at a temperature of 1000 for a period of 60 minutes. The resulting surface concentration of the wafers was 2.5 X 1 0 atoms/cc. Each of the silicon wafers had their impurity profile determined by the automatic method using the apparatus of FIGS. 4-6. The first wafer was automatically profiled using the removal of 1 micro inch of silicon per cycle. The sheet resistance and the photovoltage were recorded after each cycle according to the techniques of the present invention described above. The second silicon wafer was processed using a removal of 0.64 micro inch of silicon per cycle and the sheet resistance were recorded after each cycle. The third wafer was processed using a removal of 0.3 micro inch of silicon per cycle and the sheet resistance were recorded. FIG. 9 shows the result of the plotting of the silicon removal versus the sheet resistance for each of the three Wafers. As can be readily seen from the profile of FIG. 9 there is excellent correlation between all three of the automatic impurity profile runs regardless of the amount of material removed per cycle. FIG. 10 shows the photovoltage versus the material removed for the first silicon wafer processed. The sheet resistance versus material removed and the photovoltage versus material removed in FIGS. 9 and 10, respectively, show that the junction depth is very close to 5 micro inches in depth.

EXAMPLE 2 Two-P-type monocrystalline silicon wafers were doped with phosphorus to a surface concentration of 1 l0 atoms/ cc. by the conventional open tube diffusion process. One wafer was processed according to the automatic method of the present invention wherein there was a removal of 0.3 micro inch per cycle. The concentration of the phosphorus in atoms/cc. was obtained from the electrical resistance measurement combined with the aforementioned calculation technique, and was plotted against material removed in angstrom units. The impurity distribution of the second sample was obtained by the radio tracer neutron-activation method. This method involves the bombardment of the sample with neutrons causing the phosphorus atoms to become radioactive. Increments of material are removed by the anodizing technique and the phosphorus atoms at each increment is counted by a liquid scintillation counter. (Reference: Fundamentals of 'Silicon Integrated Device Technology, vol. 1, edited by R. M. Burger and R. P. Donovan, pages 405-407, Prentice-Hall series), FIG. 11 shows the results of this experiment. Curve 70 shows the impurity profile as determined by the automatic method of the present invention. Curve 72 shows the impurity profile determined by the neutron-activation method. The results indicate the total impurity concentration as seen from the curve 70 as only slightly higher than the electrically active impurity concentration as determined from the automatic impurity profile method, which is to be expected because it is known that because of precipitation at dislocations in silicon the electrical active impurities is less than the total impurities.

EXAMPLES 3 THROUGH 7 Junctions of various depths were made by capsule diffusions. The bevel and stain technique which involve the use of beveling the junction at a very shallow angle of about 2 degrees and preferentially staining the N and P areas, The depth was then measured by conventional optical means to obtain the junction depth in each case. For a like sample the automatic profiling impurity profile apparatus of the present invention was used. The following table gives the type of diffusion, the surface concentration of the dopant, the junction depth as determined from the bevel and stain technique and the junction depth as determined from the automatic profiling technique of the present invention.

Co i i (source), (B & S), (system), Type of difiusion atoms/cc. p in. y in.

Boron base 3X10" 14. 8 l4 Phosphorus emitter. 1X10 8. 2 10. 5 Boron base a. 3X10" 15. 7 Boron base after heating step in inert atmosphere 3X10" 12. 11. Boron base before heating step 3x10 10 9. 5

means for making said substrate the anode of paratus;

means for providing a cathode within said container;

means for periodically measuring an electrical characteristic of said semiconductor substrate;

means for controlling the input and output of anodizing solutions through said container parts, the application of an anodizing potential between said anode and cathode while the said anodizing solution is in said container, the input and output of an etching solution following the output of said anodizing solution, the control means causing the measuring means to make periodic measurements of said electrical characteristic.

2. The apparatus of claim 1 wherein said means for measuring includes a four point resistance measuring probe mounted on an arm.

3. The apparatus of claim 1 wherein said means for measuring includes a three point resistance measuring probe mounted on an arm.

4. The apparatus of claim 2 further comprising means for moving said arm into and out of said container and means for moving said arm together with said probe to allow the probe to traverse in both the x and y direction over said semiconductor substrate.

5. The apparatus of claim 1 wherein said means for supporting includes an insert which fits into said container, said insert having surfaces which bear against portions of said semiconductor substrate to hold the substrate fixed in said container.

6. The apparatus of claim 1 including means for recording said electrical characteristic in terms of the said characteristic versus the amount of material removed.

the ap- References Cited UNITED STATES PATENTS 2,913,383 11/1959 Topfer 204275X 3,048,776 8/1962 Logan 324-158X 3,134,942 5/1964 Rhodes 324158X 3,148,129 9/1964 Basseches et a1. 204228X JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 204-227, 228, 275

Patent No.

Inventor(s) UNITED STATES PATENT OFFICE Dated January 12 1 971 M. C. Duffy et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the specification:

Column Column Column Column Column line line

line

line

line

After "silicon" insert or "furgyl" should be fural Signed and sealed this 15th day of February 1972.

(SEAL) Attest:

EDWARD M.FL1:."ICHER,JR. Attesting Officer ROBERT GOTTSCHALK Commissioner of Patents 

